Automatic machines for the making of rice crackers and similar puffed or popped granular cakes by pressure-baking and expanding a food-starch containing material in a heated mold are known from the prior art to exist in a number of distinct machine variants.
Patent documents U.S. Pat. No. 4,328,741 and WO-A-88/00797, for instance, are representative for machine types using air cylinders for selectively moving upwardly or downwardly an upper baking mold relative to a movable lower baking mold and to a stationary ring mold, in which said upper mold and lower molds are coaxially receivable in a fluid-tight manner and thereby forming a sealed molding cavity. These machines produce a lot of noise and are rather energy consuming; furthermore, air cylinders are bulky devices which often are less reliable in terms of baking pressure and cracker thickness requirements.
U.S. Pat. No. 5,562,021 discloses a device for preparing puffed food products from cereals such as rice, wherein the cereal is subjected to heat and pressure in a mold cavity, defined by an exterior wall forming an interior surface about the cavity and by a mold piston or punch being reciprocally driven into and out of the cavity. To avoid the problem of seizing of the pistons while inside the mold cavity, the said patent proposes a recess of a certain depth and height formed in the interior surface of the cavity wall, and to locate therein a cylindrical insert which can move or float within the recess. The mold piston is mounted to a pressure plate, whereby an equalizing spring is provided between the top of the piston flange and the pressure plate. A hydraulic cylinder is adapted to apply pressure to the pressure plate.
U.S. Pat. No. 4,281,593 to Gevaert and Nos. 5,102,677 and 5,467,693 to Van den Berghe describe pressure-baking apparatuses comprising a hydraulic jack connected with a toggle mechanism for driving a lower mold or punch upwardly and downwardly relative to a fixed upper mold. The patents to Van den Berghe furthermore propose the use of an annular mold element defining a peripheral wall of a molding chamber which element is selectively movable relative to the fixed upper mold and also to the lower punch. This facilitates the feeding of food-starch material into a mold cavity and the removal of a puffed cracker from the pressure-baking molding expansion chamber formed by said coaxially cooperating upper, lower and peripheral mold elements.
A drawback of said machine types is the use of a toggle mechanism transmitting hydraulic power to the mold in an indirect way and requiring special care for aligning mold parts and controlling molding pressure. In addition a movable peripheral mold is more prone to overstraining and risk of mold leakage, giving rise to defective cracker quality and appearance after a period of time. In addition, the provision of a hydraulic jack together with its oil circuit, oil tank and powering motor on each production machine or unit is rather expensive.
A generally experienced inconvenience in the production of puffed-food snacks by pressure-baking and subsequently expanding a starch-containing cereal or the like mixture in a mold cavity when using currently available technology is the quantity of trash and waste crackers produced by puffing machines, which may amount to 10% or even more of total cracker output. Further disadvantages of known machines for making puffed crackers include the rather long running-in/starting-up times needed until a stable production regime is reached and the need of frequently cleaning the baking mold elements. As a result the productivity and economics of a cracker plant, in particular when making small crackers and chips, may be seriously affected.